Recent work from this and other laboratories strongly implicate the distal nephron as a critical site for the regulation of Ca and PO4 excretion. Many pathophysiological states resulting in altered rates of excretion of these ions remain largely unexplored. Thus, it is unclear if vitamin D, another Ca-PO4-regulating hormone, also participates in the renal homeostasis of Ca and PO4. Furthermore, the tubular adaptation to dietary Ca deprivation has not been defined. Since reduced levels of 1,25 diOHD3 were found in laboratory rats with spontaneous hypercalciuria (SH), which have exaggeratad rates of Ca and PO4 excretion, they serve as excellent models for addressing these issues. The overall objective of the project is to test, by in vivo micropuncture and microperfusion experiments, the hypothesis that the distal tubule is an important nephron segment exerting physiologic control over Ca and PO4 transport. This will be accomplished by: (1) defining the tubular mechanism for the hypocalciuric and antiphosphaturic effects of 1,25 diOHD3; (2) examining the tubular conservation of Ca during deprivation. (3) studying the renal leak of Ca and PO4 in SH rats and evaluating their response to 1,25 diOHD3; (4) testing the thesis that the distal effects of DOCA and insulin are voltage-dependent; (5) characterizing the interactions between luminal Na and distal tubule reabsorption of Ca and PO4 as perturbed by chlorothiazide and amiloride. In setting up the model of vitamin D deficiency the associated hypophosphatemia and hypocalcemia are avoided by careful dietary supplements, while the intestinal effect of hormone replacement is prevented (by restricting dietary Ca and PO4) to unmask the renal action. Ca transport will be studied in acute parathyroidectomized (PTX) rats. PO4 transport will be studied in chronic PTX rats, except with D deficiency when PO4 infusion will be used to increased basal excretion rate. Tubular fluid electrolytes will be analyzed by the electron probe as previously published. Data from these models will advance our concepts regarding disturbances of renal Ca and PO4 metabolism in patients with idiopathic hypercalciuria, Conn's syndrome, and vitamin D deficiency and in those ingesting a low calcium diet or being treated with hypocalciuric diuretics.